This application seeks to renew funding for studies aimed at generating murine mmunodeficiency models. The major goals of the previous application were to generate murine models of immunodeficiencies relevant to human diseases. The current application builds on these studies to further evaluate the potential of V(D)J recombination and DNA repair factors to contribute to immunodeficiencies. V(D)J recombination involves lymphocyte specific activities including the recombination activating gene 1 and 2 (RAG-1 and RAG-2) products, as well as generally expressed activities, which include non-homologous DNA end-joining (NHEJ) factors such as the DNA-dependent protein kinase and the potentially associated Artemis protein. Defects in either the RAG genes or in the Artemis gene underlie essentially all reported human TB SCIDs. In our proposed studies, we aim to continue our efforts to generate murine models of RAG-deficiencies, with a particular focus on generation of RAG mutants that may mimic the corresponding mutations that lead to complex immunodeficient phenotypes in humans. In addition, we will test our recent in vitro finding that certain RAG mutations may predispose to aberrant V(D)J recombination reactions that lead to transposition-like reactions by generating mice harboring these mutations and introducing them into NHEJ-deficient backgrounds. A second major aim of this proposal is based on our recent findings that inactivation of the Artemis gene in routine cells leads to defective V(D)J recombination and genomic instability. We will now generate Artemis-deficient mice and test the role of this factor in normal lymphocyte functions in the context of V(D)J recombination, IgH class switch recombination and somatic mutation. Finally, we will test the potential of Artemis-deficiency to synergize with cell cycle checkpoint deficiencies in predisposing to lymphoid neoplasias.